1) Field of the Invention
The present invention relates to a wavelength division element and an optical module, each of which is incorporated in an optical component used for wavelength multiplexing optical communication, and which separates multiplexed lights in a plurality of wavelength bands.
2) Description of the Related Art
Recently, a high-density wavelength multiplexing optical communication system such as a wavelength division multiplexing (“WDM”) has been introduced as an optical transmission system. To realize WDM transmission, it is required that optical components provided on an optical communication path control a plurality of wavelengths. Each of these optical components has a function of multiplexing and demultiplexing (separating) a plurality of wavelengths (see Japanese Patent Application Laid-open No. 2002-243974). This multiplexing and demultiplexing function has the following advantage. Transmission signals of different types according to wavelengths, e.g., those for telephone, the Internet, VOD (video on-demand) can be subjected to wavelength multiplexing on one optical fiber, and the wavelength-multiplexed signal can be transmitted.
FIG. 15 depicts an example of the configuration of a conventional wavelength division element that separates light signals having different wavelengths (in a 1.3-micrometer band, 1.49-micrometer band, and a 1.55-micrometer band, respectively, designated according to ITU-T recommendation G 983.3) according to respective wavelengths. It is assumed that an optical wavelength band λ1 is the 1.3-micrometer band (1310-nanometer (nm) band: 1260 nm to 1360 nm), an optical wavelength band λ2 is the 1.49-micrometer band (1490-nm band: 1480 nm to 1500 nm), and an optical wavelength band λ3 is the 1.55-micrometer band (1550-nm band: 1539 nm to 1620 nm). The wavelength division method is normally carried out on an optical path 100 using two filters 101 and 102 having different wavelength division characteristics. The filter films 101b and 102b of the filters 101 and 102 are formed on surfaces of glass substrates 101a and 102a serving as optical transmission members, respectively (see Kozo Ishiguro et al.: “Optical Thin Film”, Chapter 2: Optical Properties of Multilayer Film, Kyoritu Shuppan Co., Ltd., Feb. 25, 1985).
FIGS. 16 and 17 are graphs of the wavelength characteristics of the two filters 101 and 102 shown in FIG. 15, respectively. Each of the filters 101 and 102 possesses wavelength characteristics of a short wave pass filter (“SWPF”), and transmits short wavelengths. That is, each of the filters 101 and 102 possesses a 1×2 wavelength division function for one input and two outputs.
The filter 101, which is disposed in front with respect to the incident lights, transmits a light in the wavelength band λ1 (1.3-micrometer band) and a light in the wavelength band λ2 (1.49-micrometer band), and reflects the light in the wavelength band λ3 (1.55-micrometer band) in a direction c. The light in the wavelength band λ1 (1.3-micrometer band) and the light in the wavelength band λ2 (1.49-micrometer band) are incident on the filter 102 disposed in rear of the filter 101. The filter 102 transmits the light in the wavelength band λ1 (1.3-micrometer band), and reflects the light in the wavelength band λ2 (1.49-micrometer band) in a different direction, b. The light in the wavelength band λ1 (1.3-micrometer band) is emitted in a direction that is an extension of an incident direction of the optical path 100, relative to the filters 101 and 102.
As shown in FIG. 15, it is preferable that the direction b of transmitting the light in the wavelength band λ2 (1.49-micrometer band) and the direction (direction c) of transmitting the light in the wavelength band λ3 (1.55-micrometer band) are oriented in a direction Y, orthogonal to an optical axis X of the optical path 100. Namely, if the optical axis is the X-axis, the axis in the direction orthogonal to the optical axis is a Y-axis, and an axis in a height direction is a Z-axis, optical axis adjustments along directions of these optical axes X, Y, and Z can be normally made easily.
However, with the wavelength division element constituted as shown in FIG. 15, it is difficult to attain the wavelength characteristics of the filter 101 to accurately carry out wavelength division. As shown in FIG. 16, the filter 101 is required to possess the wavelength characteristics of transmitting the light in the wavelength band λ1 (1.3-micrometer band) and that in the wavelength band λ2 (1.49-micrometer band), and of not transmitting (reflecting) the light in the wavelength band λ3 (1.55-micrometer band). A divided wavelength width between the wavelength band λ2 (1490-nm band: 1480 nm to 1500 nm) and the wavelength band λ3 (1550-nm band: 1539 nm to 1620 nm) is as small as 39 nm. To realize such wavelength characteristics, a special manufacturing technique, to enable a rapid change in transmission characteristics between the wavelength bands λ2 and λ3, is required. Therefore, cost increases due to yield of the filter 101.
Furthermore, the conventional wavelength division element is constituted so that the two filters 101 and 102 are arranged along the optical path 100 on which a plurality of wavelength bands λ1, λ2, and λ3 are multiplexed and the wavelength-multiplexed light signals are incident. Therefore, the disadvantage in the conventional wavelength division element is that the filters need lot of space along the optical path 100, thereby increasing the size of the wavelength division element. Moreover, the number of parts cannot be reduced, and efficiency of manufacturing and assembly does not improve.